An image fixing assembly of an electrophotographic image duplicating or printing apparatus ordinarily uses a combination of a heater roller and a pressing roller held in rollable contact with the heater roller. The image fixing assembly thus composed of the heater and pressing rollers is disposed anterior, in the direction of travel of a print sheet, to an image transfer drum by means of which toner images each formed by toner particles are produced on one surface of the print sheet. The print sheet having the toner images thus formed on one surface thereof is transported past the image transfer drum to the image fixing assembly and is passed between the heater and pressing rollers.
The heater roller which forms part of the image fixing assembly has incorporated therein a heater unit which is activated or de-activated at controlled timings during each cycle of printing operation so that the heater roller is to be heated to a predetermined optimum temperature or to a temperature within a predetermined temperature range. The timings at which the heater unit is to be activated and de-activated are controlled on the basis of a signal produced from a temperature sensitive element typically implemented by a thermister. The thermister is located in proximity to the heater roller to detect the temperature of the outer peripheral surface of the roller for producing a signal which varies with the detected temperature of the heater roller.
The signal output from the thermister is supplied to a combination of a resistor bridge network including the thermister as one of the resistors and a comparator responsive to a voltage output from the bridge network. Alternatively, the signal from the thermister is supplied to a microprocessor unit and is processed on the basis of the various data which are preliminarily stored therein. An image forming apparatus using a control system for the image fixing assembly of the apparatus is disclosed in, for example, U.S. Pat. No. 4,144,835 and Japanese Provisional Patent Publication (Kokai) No. 60-115977.
In a known heater control system of the described nature, the thermister implementing the temperature sensitive element may fail to operate properly or the microprocessor unit responsive to the signal output from the temperature sensitive element may malfunction in processing the signal received. Such a malfunction of the microprocessor unit may be invited due to an ingress of a noise of a lightning surge into the microprocessor unit chip. In the event the temperature sensitive element has thus failed or the microprocessor unit is disabled from operating properly, the heater unit of the image fixing assembly may be heated excessively and may thus cause damage to the heater roller and associated members and elements or cause firing of the print sheet being passed through the fixing assembly.
An overheat of the heater roller can be to some extent prevented through provision of a fuse or other type of temperature responsive circuit breaker inserted into the power supply circuit for the heater unit. A temperature responsive circuit breaker presently available is however not fully acceptable for its response characteristics. The temperature at which the fuse is to be blown is ordinarily selected to be higher than the maximum temperature to which the heater roller may be possibly heated during normal operation of the image fixing assembly. If the heater unit is de-energized by the blowout of the fuse, it may thus happen that, at the point of time the fuse is blown out, damage has already been caused to the heater roller although the firing from the heater unit could have been successfully precluded.
Attempts have therefore been made to provide means adapted to de-activate the heater unit before the fuse or other type of temperature sensitive circuit breaker is blown in response to an unusual rise in the temperature of the heater roller. A heater control system which has thus far been proposed however has a problem in that the system is dependent solely on a signal produced by a temperature sensitive element such as a thermister. Another problem is that the system is designed on the assumption that the temperature sensitive element at all times operates in a sound state or the microprocessor unit responsive to the signal from the temperature sensitive element at all times functions properly in respect of the hardware structure and software programs.
When the temperature sensitive element of the heater control system fails to properly operate or the microprocessor unit included in the control system malfunctions, the heater control system could not correctly cope with the situations involved. It may also be noted that the temperature sensitive element is disposed to be responsive to the temperature of the heater roller at the outer peripheral surface of the roller so that, where the heater unit includes two or more heater elements, the control system could not locate the trouble from the signal produced by the temperature sensitive element.
It is, accordingly, an object of the present invention to provide an image forming apparatus in which the control system for the image fixing assembly of the apparatus operates such that whether or not the heater unit is unusually activated is determined not from the signal from the temperature sensitive element but on the basis of a signal output from, for example, an ultrared ray sensor responsive to emission of radiation from a heater element. This means that the state of the heater unit is inspected without respect to the temperature to which the heater roller is heated after the apparatus is switched in. When any failure is present in the control system so that the heater unit is unusually activated after the apparatus is switched in, the heater unit is automatically de-activated upon detection of such unusual activation of the heater unit. The heater unit could not therefore be activated unless the failure in the control system is remedied and, if the power supply switch for the apparatus is turned on repeatedly with the failure unremedied, any accident which might otherwise result from improper activation of the heater unit such as the damage of the heater roller or the firing from the roller might be caused due to an overheat of the heater roller in a prior-art image forming apparatus can be reliably precluded.
The ultrared ray sensor used as in the apparatus of this nature is one of the various detector means which further include a galvanometer responsive to the flow of current through the lines connected to the heater element or elements, a magnetic field sensor responsive to variation in the magnetic field induced by the current through such lines, and any radiation responsive detector responsive to the visible or hot-wave radiation from the heater element or elements. The radiation responsive detector may be implemented a thermister, a positive temperature coefficient thermister, or a pyroelectric-effect sensor.